GRAIL is not a single spacecraft. Launching on one Launch Vehicle, GRAIL-A and GRAIL-B are two nearly identical spacecraft that are needed to map the Moon’s interior. GRAIL features a single-string architecture that will be sufficient to support its short duration mission. 'Single string' means that there is no redundancy built into every system which doesn't allow switching to a secondary system should the primary string fail.
The vehicles are about the size of a washing machine. Each of them has a mass of 202kg (Fueled Mass: 307kg). Both spacecraft have to point antennas at each other which prompts slight differences in vehicle design. MoonKAM mounting, antenna and star tracker positioning are the only differences between the Orbiters. GRAIL-B is designed to precede GRAIL-A in lunar orbit.
The spacecraft will use systems that are currently in use on the GRACE Spacecraft that essentially does what GRAIL will do in Earth Orbit..
GRACE (Gravity Recovery And Climate Experiment) is a cooperation of NASA and the German Space Agency DLR. The twin satellites were launched in 2002 and have been returning valuable gravitational data of our home planet ever since. GRACE has led to discoveries in the fields of ocean, geology, and climate.
The Twin GRACE Spacecraft orbiting Earth
The avionics sytems of GRAIL have been derived from another NASA Mission, the Mars Reconnaissance Orbiter. Its design is based on an Experimental Satellite built by Lockheed Martin in cooperation with the US Air Force Research Laboratory (XSS-11). The XSS-11 was launched in April 2005 and operated for 1 year before being placed in a disposable orbit in which it remains to this date (August 2011).
Navigation and Attitude Control information is being provided by a sun-sensor, star tracker and one intertial measurement unit. The reaction control system of GRAIL is three-axis stabilized.
Electrical power is being generated by two solar arrays. Each solar array will be providing 700 Watts at the end of the mission to satisfy the need of all onboard systems. Each array has an area of 6.2 square feet. 26 strings of 20 sloar cells are on each panel. A failure of a number of solar cells is expected and there is plenty of margin to allow these types of problems. A lithium ion battery will provide power when the vehicles are passing through eclipse. Both solar arrays will be deployed shortly after spacecraft separation. Those will stay fixed for the entire mission. Flexible Arrays would cause movement that could interfere with science readings.
Main propulsion is provided by a Hydrazine fueled thruster that is being used for lunar orbit insertion and trajectory changes like orbital altitude variations. The main engine is a MR-106L thruster. with 22 Newtons of thrust. Eight 0.9-Newton thruster valves and a warm gas system are used for attitude maneuvers and small trajectory changes.
GRAIL's fixed Solar Arrays will only be able to receive sunlight during a limited amount of time, so the mission can not be extended.
Each vehicle will have to support several communications channels and subsystems. Every orbiter has two S-Band transponder antennas for communications with ground stations on Earth. Two X-Band beacon antennas will be used for Doppler Ranging Measurements from Earth (only when the spacecraft pass the Moon’s near side) Another S-Band based System is going to send time-synchronization codes back and forth between the vehicles. A Ka-Band ranging system will measure the distance between the spacecraft in a very precise manner. This system is the primary science payload of the mission. All antennas are protected by thermal enclosures.
Illustration of GRAIL's multi channel communication system
Each GRAIL Spacecraft will have two primary payloads. The Lunar Gravity Ranging System (LGRS) is the system that will provide all science data of the mission. MoonKAM is the second payload on GRAIL. A series of 5 cameras on each vehicle will be used for education&public outreach purposes. LGRS is based on the system that is being operated on the GRACE Satellites in Earth Orbit.
LGRS is the system that is used to precisely determine the range between the spacecraft and register even the smallest changes in range/velocity. The system consists of an Ultra Stabilized Oscillator (USO) that provides a stable reference signal that is being used by all other subsystems providing the reference frequency for other components of the LGRS. A Microwave Assembly converts the reference frequency from the USO to a Ka-Band frequency which is transmitted to the other GRAIL Orbiter. The TTA (Time Transfer Assembly) is providing a two-way time data link between the spacecraft. The system will measure clock offsets between the two LGRS systems of the spacecraft and synchronize clocks of both orbiters. TTA generates a singal from the USO frequency that is transmitted via S-Band and sends a GPS-like ranging code to the other spacecraft. The Gravity Recovery Processor Assembly combines data received from the MWA and TTA, converts it to radiometric data and sends it to Deep Space Network Ground Station on Earth.
LGRS also provides a one-way communication link to the ground that is based on USO Frequencies and transmitted via the X-Band Science Beacon (RSB). Through the one-way doppler radar, steady-state drift of the USO is being measured.